Device for detecting three-dimensional electromagnetic radiation and method for making same

ABSTRACT

A device for detecting electromagnetic radiation including at least one detection circuit including a non-cooled thermal detector and an associated reading circuit, and a method for producing the device. The device includes a substrate forming a cavity under vacuum, in which are placed the thermal detector and the reading circuit. A window transparent to radiation is placed above the cavity and ensures the sealing of the cavity. A sealing mechanism hermetically attaches the window on the substrate, and an electrical connection is inserted in the window ensuring a leaktight connection between the detection circuit and processing elements outside the cavity.

FIELD OF THE INVENTION

The invention concerns a device for detecting electromagnetic radiation,such as infrared radiation, in which the detectors and the readingcircuit are inserted in a cavity under vacuum, sealed by a windowtransparent to electromagnetic radiation, allowing leaktight electricalconnections towards the exterior.

The invention also concerns a method for producing said device.

The invention concerns the field of thermal detectors and, inparticular, non-cooled thermal infrared detectors. It may be used, forexample, in monolithic infrared imagers operating at ambienttemperature, and produced from a matrix of sensitive elements connectedto a multiplexing system in CMOS or CCD type silicon.

STATE OF THE PRIOR ART

Non-cooled thermal detectors generally comprise a sensitive element thatcan be heated by infrared radiation in the band 8 to 12 μm, saidsensitive element being characteristic of the temperature and theemissivity of the bodies observed. An increase in the temperature of thesensitive element leads to a variation in an electrical property of thesensitive material; said property may be, for example, the appearance ofelectrical charges by pyroelectric effect, or instead the variation inthe capacitance by a change in the dielectric constant, or even thevariation in the resistance when the material is a semi-conductor ormetallic.

Three main conditions are necessary for said detectors to operate in anoptimal manner. The sensitive material must have a low calorific mass,good thermal insulation of its active layer with respect to its supportand a high sensitivity to the effect of converting the heating into anelectrical signal, the first two conditions requiring the sensitivematerial to be produced as a thin layer.

In numerous applications, such as infrared imaging applications, thermaldetectors must be conditioned under vacuum or under a gas not conductiveto heat in order to improve performance. In this case, the thermaldetectors are encapsulated in a housing comprising a window that istransparent in band III, in other words in the band from 8 to 12 μm. Theclassical operation for encapsulating in a housing is difficult, interms of output, and relatively costly. To reduce the cost of saidintegration, collective encapsulation methods have been proposed.

One of said collective encapsulation methods is described in documentWO-95/17014. This involves a collective encapsulation method undervacuum (or under a gas not conductive to heat) by coupling of a wafer ofdetectors with a wafer of windows transparent to infrared radiation. Theconnection between the two wafers is made via a weld bead that ensures,firstly, the leaktightness of the assembly and, secondly, allows thepassage of the electrical connection between the interior and theexterior of the device. Said weld bead thus determines, as a function ofits thickness, the spacing between the two components of the housing.The spacing may also be achieved by a spacer generated from layersdeposited on the wafer of the device or the wafer of the window, or evendirectly produced in the material of the window. For components withlarge dimensions, a prop may be placed in the centre, in order torestrict deformation of the components with large dimensions.

The method described in said document proposes placing a metallic layeron the side of the wafer of detectors and on the side of the wafer ofwindows in order to ensure the wettability and the anchorage of theweld.

Furthermore, maintaining the vacuum within the micro-housing is ensuredby employing materials that do not have an excessive degassing rate.However, even with a relatively low degassing rate, it is virtuallyobligatory to use a getter material to absorb the gases emitted by thedifferent surfaces, because an increase in the pressure deteriorates thethermal insulation of the micro-bridges. Said getter material may bebarium, vanadium, iron, zirconium or alloys thereof. However, beforebeing active, said materials must be activated at high temperature, fora short period, either by Joule effect or by laser beam, without howeverthe detector and the window being overly heated. In order to solve thisproblem, said getter material is deposited on micro-bridges reserved forthis purpose, with the aim of confining the heating uniquely to thegetter material.

In FIG. 1A, we have schematically represented said collective method forproducing infrared detection devices.

Said FIG. 1A shows a detection wafer 2, a wafer of windows transparentto infrared radiation 3, and micro-bolometers, in other words non-cooledinfrared detectors, referenced 4 a, 4 b and 4 c. Said FIG. 1A also showsthat the detection wafer 2 and the window wafer 3 are separated atregular intervals by spacers 5 a, 5 b, 5 c, 5 d, etc., which ensure theseparation of the different detection components.

The assembly represented in FIG. 1A is the cut between two spacers, forexample between 5 b and 5 c, in order to form different infrareddetection devices.

Furthermore, an infrared detection device with a silicon window isdescribed in patent application GB-A-2 310 952. Forming the windowtransparent to infrared radiation out of silicon has advantages, namelythe low cost and good compatibility, in terms of dilation coefficient,with the detection circuit also formed on a silicon substrate. Moreover,the silicon makes it possible to obtain a good compromise between themechanical properties and the electrical properties of the device.

In order to ensure the assembly of the window with the detectioncircuit, said document proposes either carrying out an eutectic sealingor a sealing by low melting point fritted glass, or by thermocompressionwelding. In the case of eutectic sealing, it is proposed using tin/leadalloys for the sealing, and a triple W/Ni/Au layer for the anchoringmetallisations.

In FIG. 1B, we have schematically represented the above-mentionedinfrared detection device, after cutting on the exterior of spacers 5 cand 5 d.

Said infrared detection device comprises a detection circuit comprisingthe detectors 4 a and 4 b and reading circuits, and a window transparentto infrared radiation 3. Said window 3 is maintained above the detectioncircuit 2 through two spacers 5 c and 5 d. This whole device is thenattached to a support 6, to which it is connected throughinterconnecting wires 7.

This type of device requires sealing beads 5′ on the detection circuit2, which allow the passage of the electrical connection between thedetection circuit 2 and the means of processing outside the device,after electrical insulation of said connections. However, theintroduction of said beads in the manufacturing method is extremelytricky because it requires the presence of gold, which presents the riskof contaminating the CMOS circuit of the detection circuit 2.

Furthermore, said sealing bead increases the size of the detectioncircuit which constitutes the most costly element of the final device.The topology at the level of the sealing joint 5′ must be considerablyreduced, in other words it must be planarised, by mechanical/chemicalpolishing in order to ensure hermetic sealing.

Moreover, as explained above, it is necessary to use getters in saiddevice in order to ensure a vacuum is maintained within the interior ofthe device, between the detection circuit 2 and the window 3. However,said getters are placed next to the infrared detectors, furtherincreasing the surface of the detection circuit 2, to the detriment ofthe number of detection circuits per wafer.

SUMMARY OF THE INVENTION

The precise aim of the invention is to overcome the disadvantages of thedevice and the manufacturing method described previously. To this end,it proposes a device for detecting infrared radiation formed in threedimensions, in such a way as to transfer the part relating to thesealing on the elements considerably less expensive than the detectioncircuits and in such a way as to house the getter below the detectioncircuit.

More precisely, the invention concerns a device for detectingelectromagnetic radiation comprising at least one non-cooled thermaldetector and a reading circuit, characterised in that it comprises:

-   -   a substrate forming a cavity under vacuum, in which are placed        the thermal detector and the reading circuit;    -   a window transparent to radiation, placed above the cavity and        ensuring the sealing of said cavity;    -   sealing means for hermetically sealing the window on the        substrate; and    -   electrical connection means inserted in the window, ensuring a        leaktight connection between the detection circuit and the        processing elements outside the cavity.

Advantageously, the device comprises a getter placed in the cavity,below the reading circuit.

According to one embodiment of the invention, the connection means aremetallised apertures.

According to another embodiment of the invention, the connection meansare metal contact plates substantially in the form of an H and passingright through the window.

In yet another embodiment, the sealing means consists in an anodicsealing.

In a further embodiment, the sealing means consists in an eutecticsealing.

The invention also concerns a method for producing said device. Moreprecisely, it involves a method for encapsulating non-cooled thermaldetectors, combined with at least one reading circuit and intended todetect electromagnetic radiation. Said method is characterised by thefact that it consists in:

a) forming a cavity in a substrate;

b) forming apertures in a window transparent to the radiation to bedetected and inserting, in a leaktight manner, electrical connectionmeans in the apertures of the window;

c) connecting the detection circuit to the transparent window throughthe connection means; and

d) placing the detection circuit in the cavity and hermetically sealingthe transparent window on the substrate.

Advantageously, the method of the invention comprises a step c′, whichconsists in placing a getter in the cavity, before putting in place thedetection circuit.

Advantageously, an anti-reflection layer is deposited on either side ofthe transparent window.

According to one embodiment of the invention, the detectors areassembled on n reading circuits, where n≧2. In this case, the method ofthe invention consists in:

-   -   carrying out n times step a) in a wafer of substrate;    -   carrying out n times step b) in a wafer of transparent windows;    -   carrying out n times step c) in said wafer of windows;    -   carrying out step d) for each cavity; and    -   cutting the assembly formed from the window wafer and the        substrate wafer in order to obtain n detection devices        conforming to the detection device of claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A, already described, schematically represents the general step ofthe method for collectively producing the device of the prior art;

FIG. 1B represents the device of the prior art obtained by the method ofFIG. 1A;

FIG. 2 represents the device for detecting infrared radiation of theinvention; and

FIG. 3 schematically represents the different steps for producing thedevice of FIG. 2.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In FIG. 2, we have schematically represented the device for detectinginfrared radiation according to the invention. The references shown inFIG. 2, which are similar to those given in FIGS. 1A and 1B, representidentical elements.

The device of the invention is in the form of a stack of elements inthree dimensions. One of said elements is a detection circuit 2, whichcomprises a reading circuit and three non-cooled infrared detectors 4 a,4 b and 4 c, which are also called “micro-bridges” and which areconnected to the reading circuit. The number of detectors may be high(for example, 256×256).

When the detection device is applied to infrared imaging, eachmicro-bridge represents a pixel of the detected image.

The detection circuit 2 is placed within the interior of a substrate 8hollowed out in such a way as to form a cavity 10.

Another element of the stacking 3D is a window transparent to infraredradiation, referenced 3, and placed above the cavity 10, in such a wayas to ensure the sealing of the cavity 10. Said transparent window 3rests on the walls 8 a and 8 b of the cavity 10. It is attached to saidwalls by sealing, for example anodic type sealing, as represented byreference 12 in FIG. 2. Said window 3 may also be attached to the wallsof the cavity 10 by an eutectic type sealing, as represented byreference 13. It will be understood, obviously, that when producing sucha device, one carries out the sealing of the window on the substrate 8(in other words on the walls 8 a and 8 b of the cavity 10) by a sametype of sealing, namely an anodic sealing or an eutectic sealing.

The transparent window 3 is pierced by several apertures that allow thepassage of the means of connection between the exterior of the deviceand the interior of the cavity.

In the embodiment in FIG. 2, the window transparent to infraredradiation comprises two apertures, referenced 3 a and 3 b, allowing thepassage of the means of connection 9 through the window 3. The aperturesmay be metallised by a screen printing process or by thermaldecomposition (LPCVD method). Preferably, the thickness of saidmetallised layers is between 0.5 μm and 5 μm.

The means of connection 9 ensure that the detection circuit 2 ismaintained within the interior of the cavity 10.

As a result, they ensure the positioning of the infrared detectors 4 a,4 b and 4 c in relation to the window. Said means of connection 9 allow,moreover, the leaktight passage of a leaktight electrical connectionbetween the detection circuit 2 contained in the cavity 10 and the meansof processing 20 outside the device. Said external means of electronicprocessing 20 may be, for example, an electronic board 20 a connected,through the welding contacts 20 b, to one or several means of connection9 of the detection device.

The means of electrical connection 9 may be of different types.Nevertheless, it is necessary for said means of connection to besufficiently rigid to allow the mechanical attachment of the detectioncircuit 2, in order to ensure a stable position of said circuit withinthe interior of the cavity and for it to be leaktight in order to ensurea vacuum is maintained within the cavity 10. It should be noted that,throughout the description and in the claims, we will talk about the“cavity under vacuum”, it being understood that it can also involve agas with low heat conductivity.

Said means of connection 9 may, for example, consist in metal contacts 9a having a substantially reversed H form and passing right through thewindow (they will be called, in the following description, metalcontacts in H). But any technique using metallised holes totally orpartially filled are also suitable.

Said means of connection also comprise metallic connection contacts 9 c,attached to the detection circuit 2 (called flat metallic contacts).Hybridisation contacts, referenced 9 b, ensure the mechanical connectionand the electrical connection between the metal contacts in H 9 a andthe flat metallic contacts 9 c. Said contacts 9 a, 9 b and 9 c are madeout of metal materials, such as Ti, TiN, Pt, Al, Au, W, Ni, Ln, Sn,MnPb, SnPb, etc. They are deposited on the device by cathodicsputtering, CVD or by evaporation method.

The device of the invention has the advantage of comprising asufficiently deep cavity to make it possible to install a getter withinthe interior of the device, under the detection circuit 2. In FIG. 2, wehave represented a getter 11, placed on a getter support 11′, ifnecessary thermally insulating, resting on recesses 14 formed in thewalls 8 a and 8 b of the cavity. The getter thus placed below thereading circuit 2, makes it possible to save considerable space comparedto classical devices in which the getter is placed beside the readingcircuit. The getter may thus have large dimensions and thus may be usedas a micropump, in other words it may partially replace a secondarypumping device, in general on the exterior of the assembly that needs tobe pumped.

Furthermore, this positioning of the getter, opposite the rear face ofthe reading circuit 2, avoids the risks of alteration of the detectors 4a, 4 b and 4 c during the sealing, since the getter is not in directcontact with the detection circuit.

In the device of the invention, the sealing of window 3 on the substrate8 may be an anodic sealing; this implies the use of PYREX® type glass(borosilicate), heavily doped with sodium or potassium. The assembly tobe encapsulated is then placed under vacuum at a temperature typicallybetween 100 and 500° C., and in the presence of a strong electric field,of around 7×10⁶ V/m in the glass. The duration of the operation mayreach around 30 minutes. However, under the combined effect of theelectrical field and the temperature, the ions migrate towards the anodeand the cathode, where they are trapped. The ions, thus accumulated,create a strong internal electrical field, which ensures the adhesion ofthe two materials present. Nevertheless, it is known that CMOS circuits(as is the case for the reading circuit) are sensitive to the diffusionof such metal ions and to the strong electrical field. But, in theinvention, the detection circuit 2 is “suspended” in the cavity throughthe means of connection 2; the anodic sealing is therefore made betweenthe window 3 and the walls 8 a and 8 b of the cavity 10, without risk oftouching the detection circuit 2. Thus, the electrical field requiredfor the sealing is confined to the exterior of the detection circuit; ittherefore does not risk being degraded.

In the device of the invention, it is also possible to seal the window 3on the substrate 8 by an eutectic sealing. This type of sealing consistsin introducing a metallic layer, for example gold, between two siliconsurfaces, in other words a layer of gold 13 between the window and thewalls of the cavity, then heating the whole assembly. The meltingtemperature of the mixture that is formed by diffusion is lower thanthat of the metal or silicon. Thus, the gold reacts with the silicon at363° C. to form the eutectic AuSi.

In the device of the invention, the detection circuit is never directlyin contact with the metal, for example gold, used for the sealing. Thereis therefore no risk of contamination of the detection circuit by thegold.

Moreover, in said device, there is no relief arising from metallisation,in other words metallic contacts crossing through the sealing beadtowards the exterior. The eutectic sealing may thus be carried outwithout any problem between the window and the walls of the cavity.

The sealing of the window on the walls of the cavity may also be formedfrom low melting point glass or even adhesives or by adding brazingalloy.

In order to form the device of the invention, it is necessary toactivate the getter, which consists in heating it. The heating may becarried out by circulating an electrical current or even by heating thesubstrate/window assembly, or advantageously by laser. Said laseractivation is carried out through the substrate which must betransparent around the wavelength that the laser operates. The use of ascanning laser makes it possible to activate the maximum surface of thegetter 11, while at the same time avoiding overly heating the thermaldetectors. In this respect, the rear face of the detection circuit maybe equipped with a layer that reflects the infrared light emitted by thegetter when it is activated. The support 11′ of the getter is then madeout of a thermally insulating material. Said laser activation processmakes it possible to reach a temperature higher than 500° C. and, thus,to obtain an activation period of only several minutes.

The activation may also be achieved by radio frequency. In this case,the material constituting the getter 11 or its support 11′ may bemetallic. One then carries out a heating by induction from the rear faceof the substrate forming the cavity. Thus, an electromagnetic waveprovokes the circulation of induced currents, which lead to, in theirturn, losses by Joule effect due to the eddy current. In order not todegrade the detection circuit during the activation phase, the thicknessof the getter and/or its support must be adapted in such a way as toachieve an efficient shielding. As for laser activation, the rear faceof the detection circuit may comprise a reflective layer, preferablynon-conductive to electricity.

The device of the invention, as described previously, may be formed bymeans of a method called “encapsulation of non-cooled thermaldetectors”. As shown in FIG. 3, said method consists, firstly, informing a cavity 10 in a substrate 8 (step E1), said substrate being,for example, in silicon or in glass. It then consists (step E2) informing apertures 3 a and 3 b in a window transparent to infraredradiation; said window may be, for example, in silicon, in germanium oreven in ZnS. Its thickness depends on its nature and the format of thedetectors. It is between, preferably, 100 μm and 2 mm.

Means of connection 9 are then inserted into the apertures of the window(step E3), in such a way as to allow a leaktight connection.

Then, the detection circuit 2 is connected to the transparent window 3through the means of connection 9 (step E4). The detection circuit thusattached to the window 3 is then inserted into the cavity 10 (step E5).The transparent window 3 is hermetically sealed on the walls 8 a and 8 bof the substrate.

In the preferred embodiment of the invention, the method consists inplacing a getter 11 in the cavity, for example resting on recesses 14formed in the cavity, and this is done before putting in place thereading circuit. The getter may also be formed directly in the substratewhen said substrate is made to form the cavity.

An additional step makes it possible to form an anti-reflection layer onthe window, by depositing an anti-reflection layer on either side of thetransparent window.

The detector of the invention may also be formed in a collective manner.Several devices may then be produced simultaneously. In this case, it isnecessary to have formed beforehand:

-   -   a detection wafer (detection circuits) on which is connected a        plurality of non-cooled infrared detectors and their reading        circuit;    -   a wafer of infrared windows made out of Si, Ge or ZnS, with        connections towards the exterior;    -   a wafer of substrate with cavities and getters.

The wafer of windows and the wafer of substrates have been formedbeforehand, in a separate manner, using classical micro-electronictechniques. In other words, the cavities in the wafer of substrates andthe apertures in the wafer of windows are produced by chemical etchingor plasma etching methods.

Thus, when the wafer of windows or the wafer of substrates have beenformed, one hybridises the detection wafer onto the window wafer, whichhas been given an anti-reflection treatment beforehand. Then, thecomponents of the detection wafer are separated by classic means forcutting integrated circuits on wafers. The collective production methodthen consists in placing getters within the interior of the cavities andhermetically sealing the wafer of windows on which is attached thedetection wafer with the substrate wafer. Said sealing may be carriedout according to any of the previously described methods.

One then carries out the cutting of the assembly, in a single operation.The activation of the getter may be carried before or after cutting, insuch a way that at the end of the activation phase, the residualpressure in the cavity is less than 10 mTorr (=10⁻² mbar).

According to one embodiment, it is possible, in order to increase thenumber of detection circuits per wafer of substrate, to cut the wafer ofdetection circuits and to collectively connect the detection circuitsthus obtained to the window wafer and only then to place and then toseal the assembly on the substrate wafer.

1. Device for detecting electromagnetic radiation including at least onedetection circuit including a non-cooled thermal detector and anassociated reading circuit, comprising: a substrate forming a cavityunder vacuum, the thermal detector and the reading circuit being placedin the cavity; a window transparent to radiation, placed above thecavity and ensuring sealing of the cavity; sealing means forhermetically sealing the window on the substrate; and electricalconnection means inserted in the window, for ensuring a leaktightconnection between the at least one detection circuit and processingelements outside the cavity.
 2. Device according to claim 1, furthercomprising a getter placed in the cavity, below the reading circuit. 3.Device according to claim 1, wherein the electrical connection meansinclude metallized apertures.
 4. Device according to claim 1, whereinthe electrical connection means include metallic contact plates,substantially in a form of an H, passing right through the window. 5.Device according to claim 1, wherein the sealing means includes ananodic sealing.
 6. Device according to claim 1, wherein the sealingmeans includes an eutectic sealing.